Hydrothermal zeolite activation

ABSTRACT

Substitution of aluminum or gallium for boron or iron contained in the framework of a high silica content zeolite is effected by treating the zeolite in liquid water in the presence of a compound of aluminum or gallium.

CROSS REFERENCE TO RELATED APPLICATION

This invention is a continuation-in-part of copending U.S. PatentApplication Ser. No. 465,987 filed Feb. 14, 1983, the entire content ofwhich is herein incorporated by reference as if fully set forth.

FIELD OF THE INVENTION

This invention relates to a method for increasing the catalytic activityof crystalline zeolites. In particular, a novel activation process isprovided to enhance the alpha value of high-silica ZSM-5 type catalystsby hydrothermal treatment in contact with an inorganic activating agent.

BACKGROUND OF THE INVENTION

Zeolite catalysts have become widely used in the processing of petroleumand in the production of various petrochemicals. Reactions such ascracking, hydrocracking, catalytic dewaxing, alkylation, dealkylation,transalkylation, isomerization, polymerization, addition,disproportionation and other acid catalyzed reactions may be performedwith the aid of these catalysts. Both natural and synthetic zeolites areknown to be active for reactions of these kinds.

The common crystalline zeolite catalysts are the aluminosilicates suchas Zeolites A, X, Y and mordenite. Structurally each such material canbe described as a robust three dimensional framework of SiO₄ and AlO₄tetrahedra that is crosslinked by the sharing of oxygen atoms wherebythe ratio of total aluminum and silicon atoms to oxygen is 1:2. Thesestructures (as well as others of catalytic usefulness) are porous, andpermit access of reactant molecules to the interior of the crystalthrough windows formed of eight-membered rings (small pore) or oftwelve-membered rings (large pore). The electrovalence of the aluminumthat is tetrahedrally contained in the robust framework is balanced bythe inclusion of cations in the channels (pores) of the crystal.

An "oxide" empirical formula that has been used to describe the aboveclass of crystalline zeolites is

    M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O

wherein M is a cation with valence n, x has a value of from 2 to 10, andy has a value which varies the pore volume of the particular crystalunder discussion. The above oxide formula may be rewritten as a general"structural" formula

    M.sub.2/n [(AlO.sub.2).w(SiO.sub.2)]yH.sub.2 O

wherein M and y are defined as above, and wherein w has a value from 1to 5. In this representation, the composition of the robust framework iscontained within the square brackets, and the material (cations andwater) contained in the channels is outside the brackets. One skilled inthe art will recognize that x in the empirical oxide formula representsthe mole ratio of silica to alumina in the robust framework of acrystalline zeolite, and shall be referred to herein simply by theexpression in common usage, i.e. "the silica to alumina ratio". (See"Zeolite Molecular Sieves", Donald W. Breck, Chapter One, John Wiley andSons, New York, N.Y. 1974, which is incorporated herein by reference asbackground material).

With few exceptions, such as with Zeolite A wherein x=2, there are feweralumina tetrahedra than silica tetrahedra in the robust framework. Thus,aluminum represents the minor tetrahedrally coordinated constituent ofthe robust framework.

It is generally recognized that the composition of the robust frameworkmay be varied within relatively narrow limits by changing the proportionof reactants, e.g., increasing the concentration of the silica relativeto the alumina in the zeolite synthesis mixture. However, definitelimits in the maximum obtainable silica to alumina ratio are observed.For example, synthetic faujasites having a silica to alumina ratio ofabout 5.2 to 5.6 can be obtained by changing said relative proportions.However, if the silica proportion is increased above the level whichproduces the 5.6 ratio, no commensurate increase in the silica toalumina ratio of the crystallized synthetic faujasite is observed. Thus,the silica to alumina ratio of about 5.6 must be considered an upperlimit in a preparative process using conventional reagents.Corresponding upper limits in the silica to alumina ratio of mordeniteand erionite via the synthetic pathway are also observed. It issometimes desirable to obtain a particular zeolite, for any of severalreasons, with a higher silica to alumina ratio than is available bydirect synthesis. U.S. Pat. No. 4,273,753 to Chang and the referencescontained therein describe several methods for removing some of thealuminum from the framework thereby increasing the silica to aluminaratio of a crystal.

For the above zeolite compositions, wherein x has a value of 2 to 10, itis known that the ion exchange capacity measured in conventional fashionis directly proportional to the amount of the minor constituent in therobust framework, provided that the exchanging cations are not so largeas to be excluded by the pores. If the zeolite is exchanged withammonium ions and calcined to convert it to the hydrogen form, itaquires a large catalytic activity measured by the alpha activity testfor cracking n-hexane, which test is more fully described below. And,the ammonium form of the zeolite desorbs ammonia at elevated temperaturein a characteristic fashion.

Synthetic zeolites wherein x is greater than 12, which have little orsubstantially no aluminum content, are known. Such zeolites have manyimportant properties and characteristics and a high degree of structuralstability such that they have become candidates for use in variousprocesses including catalytic processes. Materials of this type areknown in the art and include high silica content aluminosilicates, suchas ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-11 (U.S. Pat. No. 3,709,979),and ZSM-12 (U.S. Pat. No. 3,832,449) to mention a few. Unlike thezeolites described above wherein x=2 to 5, the silica to alumina ratiofor at least some of the high silica content zeolites is unbounded.ZSM-5 is one such example wherein the silica to alumina ratio is atleast 12. U.S. Pat. No. 3,941,871 discloses a crystalline metalorganosilicate essentially free of aluminum and exhibiting an X-ray ofdiffraction pattern characteristic of ZSM-5 type aluminosilicates. U.S.Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe microporouscrystalline silicas or organosilicates wherein the alumina contentpresent is at impurity levels. Some of the high silica content zeolitescontain boron or iron which is not reversibly removed by simple ionexchange, i.e. the zeolites contain tenaciously bound boron or iron.

Because of the extremely low alumina content of certain high silicacontent zeolites, their catalytic activity is not as great as materialswith a higher alumina content. Therefore, when these materials arecontacted with an acidic solution and thereafter are processed in aconventional manner, they are not as catalytically active as theirhigher alumina content counterparts.

It is an object of the present invention to provide a method forincreasing the catalytic activity of a high silica content zeolite thatcontains tenaciously bound boron or iron. It is a further object of thisinvention to provide a method for substituting aluminum or gallium forboron or iron contained in the robust framework of a high silica contentzeolite. It is a further object of this invention to provide novelcatalytic compositions prepared by the method of this invention.

SUMMARY OF THE INVENTION

A unique crystalline zeolite material having enhanced cracking activity(alpha-value) has been discovered by the technique of hydrothermallytreating, in the presence of a compound of aluminum or gallium, a highsilica content zeolite that contains tenaciously held boron or iron toeffect substitution by aluminum or gallium for said boron or iron.

The technique is particularly advantageous for treating hydrogen form orammonium form zeolites that have a silica to alumina ratio greater than100 to 1, and that have a boron or iron content of at least 0.1 wt%.

The novel process of this invention permits the preparation of highsilica content zeolites which have all the desirable propertiesinherently possessed by such high silica materials, and yet have an acidcracking activity (alpha-value) which heretofore has only been possibleto achieve with materials having a higher aluminum content in the robustframework.

DESCRIPTION OF PREFERRED EMBODIMENTS p As has heretofore been stated,the noval process of this invention is concerned with changing thecomposition of the robust framework of a high silica content zeolitethat contains at least 0.1 wt% of tenaciously held boron or iron. Theexpression "high silica content" is intended herein to define acrystalline zeolite structure which has a silica to alumina ratiogreater than 20 and more preferably greater than 100 up to and includingthose highly siliceous materials where the silica to alumina ratio ifvery large, e.g. greater than 1000. This latter group of highlysiliceous materials is exemplified by U.S. Pat. Nos. 3,941,871,4,061,724, 4,073,865 and 4,104,294 wherein the materials are preparedfrom forming solutions to which no deliberate addition of aluminum wasmade. However, trace quantities of aluminum are usually present due tothe impurity of the reactant solutions.

The preferred high silica content zeolite that is to be activated by theprocess of this invention has the crystal structure of a zeolite of theZSM-5 type as evidenced by X-ray diffraction. This type of zeolitefreely sorbs normal hexane, and has a pore size intermediate between thesmall pore zeolites such as Linde A and the large pore zeolites such asLinde X, the pore windows in the crystals being formed of 8-memberedrings. The crystal framework densities of this type zeolite in the dryhydrogen form is not less than 1.6 grams per cubic centimeter. It isalso known that ZSM-5 type zeolites exhibit constrained access to singlymethyl-branched paraffins, and that this constrained access can bemeasured by cracking a mixture of n-hexane and 3-methylpentane andderiving therefrom a "Constraint Index". ZSM-5 type zeolites exhibit aConstraint Index of 1 to 12 provided they have sufficient catalyticactivity or are activated by the method of this invention to impart suchactivity. The boron containing and iron containing ZSM-5 type zeolitesuseful for the process of this invention have a crystal structureexemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48.Columns 4, line 30 to column 11, line 26 inclusive of U.S. Pat. No.4,385,195 issued May 24, 1983, and the U.S. patents referred to therein,are incorporated herein by reference for a detailed descriptionincluding the X-ray diffraction patterns of the foregoing ZSM-5 typezeolites; a detailed description of crystal density and method formeasuring this property; a a detailed description of Constraint Indexand method for measuring this property; and, for matter related to theforegoing.

Methods for preparing high silica content zeolites that containtenaciously held boron or iron are known in the art and are notconsidered part of the present invention. The amount of boron containedtherein, for example, may be made to vary by incorporating differentamounts of borate ion in a ZSM-5 forming solution. One such recipe isshown, for example, by Examples 6 of European Pat. No. 68,796. Prior toactivation by treatment with aluminum or gallium by the method of thisinvention, the chosen zeolite is calcined and converted by ion exchangeto the ammonium or to the hydrogen form by calcination, by methods knownto those skilled in the art. Although either the ammonium or thehydrogen form may be activated, the hydrogen form is preferred since itis somewhat more effective. For purposes of the present invention, thezeolite must contain at least about 0.1 wt % boron or iron, although itmay contain from 0.1 wt% to about 2.5 wt%. In general, under comparableconditions, the higher the initial content of tenaciously held boron oriron, the greater the degree of substitution and of enhancement ofcatalytic activity.

The ammonium or hydrogen form of the high silica content zeolite istreated in a liquid water medium with a source of aluminum or gallium toinduce substitution and activation. The treatment is conducted at anelevated temperature of about 50° C. to 375° C. under ambient orautogenous pressure so as to maintain the water in liquid phase, and fora time effective to induce the desired extent of substitution. Dependingon the nature of the aluminum or gallium source, and depending on thetemperature, effective substitution is achieved in from about 0.25 to150 hours.

Although aluminum or gallium salts such as chlorides, sulfates andnitrates may be used, it is preferred to use the solid chalcogenides ofthese metals. Particularly useful are the various sesquioxides, such asalpha alumina monohydrate, and gel precursors of the sesquioxides. Thesolid oxide may be in the form of distinct particles, or it may becomposited with the zeolite as binder. For purposes of the presentinvention, the preferred treating material is aluminum in the form of asolid oxide and a particularly preferred embodiment is the use of alphaalumina monohydrate binder, composited with the zeolite to be treated.In general, a large excess of the treating material is used to effectthe substitution.

In general, after completion of the substition treatment, it isdesirable to ion exchange the zeolite with an ammonium salt and toconvert this to the hydrogen form by calcination prior to use of theproduct as catalyst.

While not wishing to be bound by theory, it is believed that theeffectiveness of this invention is a result of the substitution ofaluminum or gallium for boron or iron contained in the robust frameworkof the zeolite. Whereas either framework boron, for example, orframework aluminum, would be expected (if in the trivalent state) to beassociated with interstitial cations such as hydrogen ions, thoseassociated with boron have a very low or an undetectable catalyticactivity for cracking n-hexane under conditions at which hydrogen ionsassociated with aluminum have a very large activity. As is known in theart, the acid catalytic activity of a zeolite may be measured by its"alpha value", which is the ratio of the rate constant of a test samplefor cracking normal hexane to the rate constant of a standard referencecatalyst. Thus, an alpha value=1 means that the test sample and thestandard reference have about the same activity. The alpha test isdescribed in U.S. Pat. No. 3,354,078 and in The Journal of Catalysis,Vol. IV, pp. 527-529 (August 1965).

As will be seen in the examples which follow, although the method ofthis invention results in some increase in catalytic activity even whenno boron is present, the presence of increasing amounts of tenaciouslybound boron results in progressively larger increases of activity.

EXAMPLES

The following examples are for the purpose of illustrating thisinvention, and are not intended to limit the scope thereof, which scopeis defined by this entire specification including the claims appendedthereto. All parts and proportions are by weight unless explicitelystated to be otherwise. All alpha values reported in these examplesrefer to measurements made with the sample in the hydrogen form.

EXAMPLE 1

A high silica content ZSM-5 that contained tenaciously held boron wasprepared by the method described in U.S. Pat. No. 4,269,813. A portionof the product was evaluated for cracking activity and was found to havean alpha value of 7.

Another portion of the product was converted to the hydrogen form andmixed with an equal part of gamma alumina beads. The mixture washydrothermally treated in liquid water at 205° C. for 18 hours. Theproduct zeolite was separated from the beads, ammonium exchanged andcalcined. Its alpha value was found to be 12.

EXAMPLES 2-7

Six different ZSM-5 preparations with a low content of alumina weremade. They were prepared to contain from 0 wt% up to 0.95 wt% boron.Each of the products was found to have the X-ray diffration pattern ofZSM-5.

A portion of each of the products was analyzed for aluminum and boroncontent. The results are summarized in Table 1. The alpha values ofthese materials before hydrothermal treatment are shown in FIG. l.

                  TABLE 1                                                         ______________________________________                                        Example        Al, ppm  B, wt %                                               ______________________________________                                        2              677      0.00                                                  3              640      0.37                                                  4              604      0.42                                                  5              527      0.54                                                  6              670      0.58                                                  7              600      0.95                                                  ______________________________________                                    

EXAMPLES 8-11

A portion of the products of Examples 2, 5, 6 and 7 were calcined in airand base exchanged with ammonium acetate solution to convert to theammonium form. Each sample was then placed in a 30 ml screw-cap OakRidge type teflon centrifuge tube, and an equal weight of 1.5 mm gammaalumina beads were added. The samples were covered with about 20 ml ofwater and placed in a 500 ml Autoclave Engineers Zipperclave withstirrer removed, and heated to 155° C. for 65 hours under autogeneouspressure. The zeolite crystals were separated from the alumina beads,exchanged with 1 N NH₄ NO₃ at 25° C. for 18 hours, washed with distilledwater and calcined at 538° C. for 30 minutes. The zeolites were thentested for n-hexane cracking activity using the alpha test. Results aresummarized in Figure 1, which plots alpha vs. wt% B (boron) in theparent material.

EXAMPLES 12-16

A portion of each of the products of Examples 2, 3, 4, 6, and 7 weretaken to provide materials for Examples 12-16, respectively, thencalcined in air, converted to the ammonium form and calcined to convertthe ammonium form to the hydrogen form.

The hydrogen form samples were treated in the same manner as describedfor Examples 8-11, and the alpha values determined. The results areshown in FIG. 1, in increasing order of boron content of the parentmaterials.

EXAMPLE 17

Portions of the products of Examples 7 and 16 were converted to theammonium form and subjected to temperature-programmed desorption. Theresults are shown in FIG. 2.

As shown, in the drawing, the untreated sample that contained boronexhibits only low-temperature desorption, ascribable to framework boron,while the treated sample shows a large high-temperature peak, ascribableto framework aluminum, and a small low-temperature peak ascribable toresidual framework boron.

EXAMPLE 18

A portion of the product of Example 7, was treated as in Example 16except that a saturated aqueous gallium chloride solution was usedinstead of the alumina beads. The product, in the hydrogen form, wasfound to have an alpha value above 330.

What is claimed is:
 1. A method for substituting aluminum or gallium forboron or iron contained in the robust framework of a high silica contentcrystalline zeolite, which method comprises:hydrothermally treating saidzeolite in the presence of a compound of said alumina or gallium, saidhydrothermal treatment being under conditions effective to induce saidsubstitution.
 2. The method described in claim 1 wherein said compoundof aluminum or gallium is water soluble.
 3. The method described inclaim 1 wherein said compound of aluminum or gallium is an oxide.
 4. Themethod described in claim 3 wherein said aluminum is an oxide bindercomposited with said zeolite.
 5. A method for substituting aluminum forboron or iron contained in the robust framework of a crystalline zeoliteof the ZSM-5 type, which method comprises:hydrothermally treating theammonium form or the hydrogen form of said zeolite in the presence of areactive aluminum oxide and under conditions effective to induce saidsubstitution.
 6. The method described in claim 5 wherein saidcrystalline zeolite is ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 orZSM-48.
 7. A method for increasing the catalytic acid activity of a highsilica content crystalline zeolite that contains tenaciously held boronor iron which method comprises:contacting said zeolite with a compoundof aluminum or gallium under hydrothermal conditions, which conditionsinclude a temperature of 50° C. to 375° C. for from 0.25 hour to 350hours whereby enhancing said catalytic acid activity as determined bythe alpha test.
 8. The product formed by the method of claim 1 or 2 or 3or 4 or 5 or 6 or 7.